In the fields of medical diagnosis, industrial non-destructive inspection and the like, inspections using a radiographic examination apparatus such as an X-ray computed tomography apparatus (hereinafter referred to as the X-ray CT apparatus) or the like are in practice. The X-ray CT apparatus emits a fan-shaped X-ray beam from an X-ray tube to an object, collects X-ray absorption data transmitted through the object by an X-ray detector, analyzes the X-ray absorption data by a computer and reconstructs a tomogram of the object.
For the X-ray detector of the X-ray CT apparatus, there is used a solid state scintillator which, when excited by X-rays, emits visible light or the like. For such a solid state scintillator, it is considered to apply a ceramic scintillator which consists of a sintered body of a rare earth oxysulfide phosphor such as gadolinium oxysulfide, lanthanum oxysulfide, lutetium oxysulfide or the like activated by praseodymium (Pr), terbium (Tb), europium (Eu) or the like (see Patent Documents 1 to 3). Especially, gadolinium oxysulfide phosphor (Gd2O2S:Pr or the like), which is excellent in luminous efficiency and has characteristics such as a short afterglow and the like, is in practice as a scintillator material for the X-ray detector.
The X-ray CT apparatus is desired to have a higher resolution. For example, a conventional X-ray CT apparatus is impossible to realize it, and it is demanded to provide an image of alveoli. To provide the X-ray CT apparatus with a higher resolution, there is a tendency to make the X-ray detecting element more compact, and it is necessary to fabricate a ceramic scintillator in smaller size. Therefore, there is a situation that the gadolinium oxysulfide phosphor does not always have sufficient X-ray absorption. When the X-ray absorption by the scintillator is insufficient, X-ray photon noises are produced, and the quality of the X-ray CT image is deteriorated considerably.
Accordingly, a lutetium oxysulfide phosphor (Lu2O2S: Pr, Lu2O2S: Tb, Lu2O2S: Eu or the like), which has a large X-ray absorption coefficient and provides a sufficient luminous efficiency even when the ceramic scintillator is fabricated in smaller size, is being watched with interest as a ceramic scintillator material for a next-generation X-ray CT apparatus. The lutetium oxysulfide phosphor is now attempted to be produced by a flux method in the same manner as the gadolinium oxysulfide phosphor or the like. But, the lutetium oxysulfide phosphor is poor in crystal growth ability, and it is therefore necessary to add a large amount of flux (a crystal growth agent such as A3PO4 or A2CO3 (A: an alkali metal element)) in comparison with the gadolinium oxysulfide phosphor or the like.
The lutetium oxysulfide phosphor produced by using a relatively large amount of flux, which is excellent in crystalline property and has a relatively uniform grain diameter, is considered to be suitable as a ceramic scintillator forming material. But, the lutetium oxysulfide phosphor powder applying a large amount of flux has a drawback that translucency is easily impaired because coloring occurs when the ceramic scintillator (a sintered body of a lutetium oxysulfide phosphor) is produced by applying, for example, a hot pressing method, a HIP (hot isostatic pressing) process or the like.
The coloring of the ceramic scintillator and a degradation in translucency due to the coloring cause a decrease of light emission output, resulting in impairing a characteristic such as a high light emission rate which is based on a large X-ray absorption coefficient inherent in the lutetium oxysulfide phosphor. Thus, the conventional ceramic scintillator does not utilize sufficiently characteristics such as high luminous efficiency and the like on the basis of the large X-ray absorption coefficient inherent in the lutetium oxysulfide phosphor.
It is described in Patent Literature 2 that the provision of a sintered body of the rare earth oxysulfide phosphor with a high density is promoted by adjusting the amount of PO4 residue of a ceramic scintillator formed of the rare earth oxysulfide phosphor to 50 ppm or less. But, simple reduction of the amount of phosphoric acid in the sintered body (a sintered body (R: Y, Gd, La, Lu) of (R1-x-yPrxCey)2O2S phosphor) of the rare earth oxysulfide phosphor cannot enhance with good reproducibility the luminous efficiency of the ceramic scintillator applying the lutetium oxysulfide phosphor.
Meanwhile, Patent Literature 3 describes a rare earth oxysulfide phosphor ((R1-xREx)2O2S phosphor (R: Y, Gd, La, Lu, RE: Tb, Eu, Tm, Pr)) containing at least one element selected from Cs and Rb in a range of 0.2 to 50 ppm. It enhances a filling density of phosphor particles at the time of producing a radiological image conversion sheet by improving the grain shape of rare earth oxysulfide phosphor powder by Cs or Rb, and does not intend to improve the translucency of the ceramic scintillator (a sintered body of the rare earth oxysulfide phosphor).
Patent Document 1: Japanese Patent Laid-Open Application No. HEI 7-238281
Patent Document 2: Japanese Patent Laid-Open Application No. HEI 9-202880
Patent Document 3: Japanese Patent Laid-Open Application No. 2001-131546